In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. This is particularly true for cinematic projection systems. To provide a competitive alternative to conventional cinematic-quality projectors, digital projection apparatus, provide high resolution, wide color gamut, high brightness (>10,000 screen lumens), and frame-sequential system contrast ratios exceeding 1,000:1.
The most promising solutions for digital cinema projection employ one of two types of spatial light modulators as image forming devices. The first type of spatial light modulator is the digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. No. 5,914,818 (Tejada et al.); U.S. Pat. No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951 (Anderson); and U.S. Pat. No. 6,089,717 (Iwai). Although DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut, current resolution limitations (1024×768 pixels) and high component and system costs have restricted DMD acceptability for high-quality digital cinema projection.
The second type of spatial light modulator used for digital projection is the liquid crystal device (LCD). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. At high resolution, large area LCDs can be fabricated more readily than DMDs. LCDs are a viable alternative modulator technology to be used in digital cinema projection systems. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat. No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,121 (Maki et al.); and U.S. Pat. No. 6,062,694 (Oikawa et al.). Recently, JVC demonstrated an LCD-based projector capable of high-resolution (providing 2,000×1280 pixels), high frame sequential contrast (in excess of 1000:1), and high light throughput (nominally, up to 12,000 lumens). This system utilized three vertically aligned (VA) (also referred as homeotropic) LCDs (one per color) driven or addressed by cathode ray tubes (CRTs). While this system demonstrated the potential for an LCD based digital cinema projector, system complexity and overall reliability remain concerns. In addition, cost considerations render such a system not yet ready for broad commercialization in the digital cinema projection market.
JVC has also developed a new family of vertically aligned LCDs, which are directly addressed via a silicon backplane (LCOS), rather than indirectly by a CRT. While these new devices are promising, they have not yet been demonstrated to fully meet the expectations for digital cinema presentation. The JVC LCD devices are described, in part, in U.S. Pat. No. 5,652,667 (Kuragane); U.S. Pat. No. 5,767,827 (Kobayashi et al.); and U.S. Pat. No. 5,978,056 (Shintani et al.) In contrast to early twisted nematic or cholesteric LCDs, vertically aligned LCDs promise to provide much higher modulation contrast ratios (in excess of 2,000:1). U.S. Pat. No. 5,620,755 (Smith et al.), also assigned to JVC, specifically describes a methodology for inducing vertical alignment in LC displays. It is instructive to note that, in order to obtain on screen frame sequential contrast of 1,000:1 or better, the entire system must produce >1000:1 contrast, and both the LCDs and any necessary polarization optics must each separately provide ˜2,000:1 contrast. Notably, while polarization compensated vertically aligned LCDs can provide contrast >20,000:1 when modulating collimated laser beams, these same modulators may exhibit contrasts of 500:1 or less when modulating collimated laser beams without the appropriate polarization compensation. Modulation contrast is also dependent on the spectral bandwidth and angular width (F#) of the incident light, with contrast generally dropping as the bandwidth is increased or the F# is decreased. Modulation contrast within LCDs can also be reduced by residual de-polarization or mis-orienting polarization effects, such as thermally induce stress birefringence. Such effects can be observed in the far field of the device, where the typically observed “iron cross” polarization contrast pattern takes on a degenerate pattern.
As is obvious to those skilled in the digital projection art, the optical performance provided by LCD based electronic projection system is, in large part, defined by the characteristics of the LCDs themselves and by the polarization optics that support LCD projection. The performance of polarization separation optics, such as polarization beamsplitters, pre-polarizers, and polarizer/analyzer components is of particular importance for obtaining high contrast ratios. The precise manner in which these polarization optical components are combined within a modulation optical system of a projection display can also have significant impact on the final resultant contrast.
The most common conventional polarization beamsplitter solution, which is used in many projection systems, is the traditional MacNeille prism, disclosed in U.S. Pat. No. 2,403,731. This device has been shown to provide a good extinction ratio (on the order of 300:1). However, this standard prism operates well only with incident light over a limited range of angles (a few degrees). Because the MacNeille prism design provides good extinction ratio for one polarization state only, a design using this device must effectively discard half of the incoming light when this light is from an unpolarized white light source, such as from a xenon or metal halide arc lamp.
Conventional glass polarization beamsplitter design, based on the MacNeille design, has other limitations beyond the limited angular response, which could impede its use for digital cinema projection. In particular, bonding techniques used in fabrication or thermal stress in operation, can cause stress birefringence, in turn degrading the polarization contrast performance of the beamsplitter. These effects, which are often unacceptable for mid range electronic projection applications, are not tolerable for cinema projection applications. The thermal stress problem has recently been improved upon, with the use of a more suitable low photo-elasticity optical glass, disclosed in U.S. Pat. No. 5,969,861 (Ueda et al.), which was specially designed for use in polarization components. Unfortunately, high fabrication costs and uncertain availability limit the utility of this solution. Furthermore, while it would be feasible to custom melt low-stress glass prisms suited to each wavelength band in order to minimize stress birefringence, while somewhat expanding angular performance, such a solution is costly and error-prone. As a result of these problems, the conventional MacNeille based glass beamsplitter design, which is capable of the necessary performance for low to mid-range electronic projection systems, operating at 500-5,000 lumens with approximately 800:1 contrast, likely falls short of the more demanding requirements of full-scale commercial digital cinema projection.
Other polarization beamsplitter technologies have been proposed to meet the needs of an LCD based digital cinema projection system. For example, the beamsplitter disclosed in U.S. Pat. No. 5,912,762 (Li et al.), which comprises a plurality of thin film layers sandwiched between two dove prisms, attempts to achieve high extinction ratios for both polarization states. Theoretically, this beamsplitter device is capable of extinction ratios in excess of 2,000:1. Moreover, when designed into a projection system with six LCDs (two per color), this prism could boost system light efficiency by allowing use of both polarizations. However, size constraints and extremely tight coating tolerances present significant obstacles to commercialization of a projection apparatus using this beamsplitter design.
As another conventional solution, some projector designs have employed liquid-immersion polarization beamsplitters. Liquid-filled beamsplitters (see U.S. Pat. No. 5,844,722 (Stephens), for example) have been shown to provide high extinction ratios needed for high-contrast applications and have some advantages under high-intensity light conditions. However, these devices are costly to manufacture, must be fabricated without dust or contained bubbles and, under conditions of steady use, have exhibited a number of inherent disadvantages. Among the disadvantages of liquid-immersion polarization beamsplitters are variations in refractive index of the liquid due to temperature, including uneven index distribution due to convection. Leakage risk presents another potential disadvantage for these devices.
Wire grid polarizers have been in existence for many years, and were primarily used in radio-frequency and far infrared optical applications. Use of wire grid polarizers with visible spectrum light has been limited, largely due to constraints of device performance or manufacture. For example, U.S. Pat. No. 5,383,053 (Hegg et al.) discloses use of a wire grid beamsplitter in a virtual image display apparatus. In the Hegg et al. disclosure, an inexpensive wire grid beamsplitter provides high light throughput efficiency when compared against conventional prism beamsplitters. The polarization contrast provided by the wire grid polarizer of Hegg et al. is very low (6.3:1) and unsuitable for digital projection. A second wire grid polarizer for the visible spectrum is disclosed in U.S. Pat. No. 5,748,368 (Tamada). While the device discussed in this patent provides polarization separation, the contrast ratio is inadequate for cinematic projection and the design is inherently limited to rather narrow wavelength bands.
Recently, as is disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.); U.S. Pat. No. 6,243,199 (Hansen et al.); and U.S. Pat. No. 6,288,840 (Perkins et al.), high quality wire grid polarizers and beamsplitters have been developed for broadband use in the visible spectrum. These new devices are commercially available from Moxtek Inc. of Orem, Utah. While existing wire grid polarizers, including the devices described in U.S. Pat. Nos. 6,122,103 and 6,243,199 may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices do have a number of advantages. When compared against standard polarizers, wire grid polarization devices exhibit relatively high extinction ratios and high efficiency. Additionally, the contrast performance of these wire grid devices also has broader angular acceptance (NA or numerical aperture) and more robust thermal performance with less opportunity for thermally induced stress birefringence than standard polarization devices. Furthermore, the wire grid polarizers are robust relative to harsh environmental conditions, such as light intensity, temperature, and vibration. These devices perform well under conditions of different color channels, with the exception that response within the blue light channel may require additional compensation.
Wire grid polarization beamsplitter (PBS) devices have been employed within some digital projection apparatus. For example, U.S. Pat. No. 6,243,199 (Hansen et al.) discloses use of a broadband wire grid polarizing beamsplitter for projection display applications. U.S. Pat. No. 6,234,634 (also to Hansen et al.) discloses a wire grid polarizing beamsplitter that functions as both polarizer and analyzer in a digital image projection system. U.S. Pat. No. 6,234,634 states that very low effective F#'s can be achieved using wire grid PBS, with some loss of contrast, however. Notably, U.S. Pat. No. 6,234,634 does not discuss how polarization compensation may be used in combination with wire grid devices to reduce light leakage and boost contrast for fast optical systems operating at low F#'s.
In general, wire grid polarizers have not yet been satisfactorily proven to meet all of the demanding requirements imposed by digital cinema projection apparatus, although progress is being made. Deficiencies in substrate flatness, in overall polarization performance, and in robustness at both room ambient and high load conditions have limited commercialization of wire grid polarization devices for cinematic projection.
Of particular interest and relevance for the apparatus and methods of the present invention, it must be emphasized that individually neither the wire grid polarizer, nor the wire grid polarization beamsplitter, provide the target polarization extinction ratio performance (nominally >2,000:1) needed to achieve the desired projection system frame sequential contrast of 1,000:1 or better, particularly at small F#'s (<F/3.5). Rather, both of these components provide less than ˜1,200:1 contrast under the best conditions. Significantly, performance falls off further in the blue spectrum. Therefore, to achieve the desired 2,000:1 contrast target for the optical portion of the system (excluding the LCDs), it is necessary to utilize a variety of polarization devices, including possibly wire grid polarization devices, in combination within a modulation optical system of the projection display. However, the issues of designing an optimized configuration of polarization optics, including wire grid polarizers, in combination with the LCDs, color optics, and projection lens, have not been completely addressed either for electronic projection in general, or for digital cinema projection in particular. Moreover, the prior art does not describe how to design a modulation optical system for a projection display using both LCDs and wire grid devices, which further has polarization compensators to boost contrast.
There are numerous examples of polarization compensators developed to enhance the polarization performance of LCDs generally, and vertically aligned LCDs particularly. In an optimized system, the compensators are simultaneously designed to enhance the performance of the LCDs and the polarization optics in combination. These compensators typically provide angular varying birefringence, structured in a spatially variant fashion, to affect polarization states in portions (within certain spatial and angular areas) of the transiting light beam, without affecting the polarization states in other portions of the light beam. Polarization compensators have been designed to work with LCDs generally, but also vertically aligned LCDs in particular. U.S. Pat. No. 4,701,028 (Clerc et al.) discloses birefringence compensation designed for a vertically aligned LCD with restricted thickness. U.S. Pat. No. 5,039,185 (Uchida et al.) discloses a vertically aligned LCD with compensator comprising at least two uniaxial or two biaxial retarders provided between a sheet polarizer/analyzer pair. U.S. Pat. No. 5,298,199 (Hirose et al.) discloses the use of a biaxial film compensator correcting for optical birefringence errors in the LCD, used in a package with crossed sheet polarizers, where the LCD dark state has a non-zero voltage (a bias voltage). U.S. Pat. No. 6,081,312 (Aminaka et al.) discloses a discotic film compensator which is designed to optimize contrast for a voltage ON state of the VA LCD. By comparison, U.S. Pat. No. 6,141,075 (Ohmuro et al.) discloses a VA LCD compensated by two retardation films, one with positive birefringence and the other with negative birefringence.
U.S. Pat. No. 5,576,854 (Schmidt et al.) discloses a compensator constructed for use in projector apparatus using an LCD with the conventional MacNeille prism type polarization beamsplitter. This compensator comprises a ¼ wave plate for compensating the prism and an additional 0.02 λ's compensation for the inherent LCD residual birefringence effects. U.S. Pat. No. 5,619,352 (Koch et al.) discloses compensation devices, usable with twisted nematic LCDs, where the compensators have a multi-layer construction, using combinations of A-plates, C-plates, and O-plates, as needed.
In general, most of these prior art compensator patents assume the LCDs are used in combination with sheet polarizers, and correct for the LCD polarization errors. However, polarization compensators have also been explicitly developed to correct for non-uniform polarization effects from the conventional Polaroid type dye sheet polarizer. The dye sheet polarizer, developed by E. H. Land in 1929 functions by dichroism, or the polarization selective anistropic absorption of light. Compensators for dye sheet polarizers are described in Chen et al. (J. Chen, K.-H. Kim, J.-J. Kyu, J. H. Souk, J. R. Kelly, P. J. Bos, “Optimum Film Compensation Modes for TN and VA LCDs”, SID 98 Digest, pgs. 315-318.), and use a combination A-plate and C-plate construction. The maximum contrast of the LCD system aimed at in prior art patents such as in U.S. Pat. No. 6,141,075 (Ohmuro et al.) is only up to 500:1, which is sufficient for many applications, but does not meet the requirement of digital cinema projection.
While this prior art material extensively details the design of polarization compensators used under various conditions, compensators explicitly developed and optimized for use with wire grid polarizers are not disclosed. Furthermore, the design of polarization compensators to enhance the performance of a modulation optical system using multiple wire grid polarizer devices, or using multiple wire grid devices in combination with vertically aligned LCDs, have not been previously disclosed. Without compensation, the wire grid polarization beamsplitter provides acceptable contrast when incident light is within a low numerical aperture. However, in order to achieve high brightness levels, it is most advantageous for an optical system to have a high numerical aperture (>˜0.13), so that it is able to gather incident light at larger oblique angles. The conflicting goals of maintaining high brightness and high contrast ratio present a significant design problem for polarization components. Light leakage in the OFF state must be minimal in order to achieve high contrast levels. Yet, light leakage is most pronounced for incident light at the oblique angles required for achieving high brightness.
Compensator requirements for wire grid polarizing beamsplitter devices differ significantly from more conventional use of compensators with polarizing beamsplitter devices based on the MacNeille prism design as was noted in reference to U.S. Pat. No. 5,576,854. Performance results indicate that the conventional use of a ¼ wave plate compensator is not a solution and can even degrade contrast ratio. Additionally, while compensators have previously been specifically developed to work in tandem with VA LCDs in projection display systems, compensators optimized for use with VA LCDs in the context of a modulation optical system which utilizes wire grid polarization beamsplitters have not been developed and disclosed.
Thus it can be seen that there is a need for an improved projection apparatus that uses wire grid polarization devices, vertically aligned LCDs, and polarization compensators in combination to provide high-contrast output.